The present disclosure relates to nitride semiconductor devices, and more particularly to a semiconductor device using a nitride semiconductor that is applicable as a power transistor etc. particularly used for a power supply circuit.
A nitride semiconductor typified by gallium nitride (GaN) attracts attention as a material for a high-frequency semiconductor device or a high-power semiconductor device. A silicon (Si) substrate etc. is under study as a substrate on which a semiconductor device using a nitride semiconductor is disposed. A Si substrate is easy to increase in diameter, and thus a Si substrate on which a nitride semiconductor is grown can dramatically decrease the cost of a semiconductor device using a nitride semiconductor.
The potential of a Si substrate on which a semiconductor device using a nitride semiconductor is disposed affects the device operation. To stabilize the potential of the Si substrate, a back electrode is disposed on the back of the Si substrate, and connected with a source electrode or a drain electrode. When the back electrode is electrically connected with the source electrode or the drain electrode, a high voltage is applied between the back electrode and the source electrode or the drain electrode. Thus, there is a need for an increase in a vertical breakdown voltage of the semiconductor device. The vertical breakdown voltage of the semiconductor device disposed on the Si substrate depends on the breakdown voltage of the nitride semiconductor layer grown on the Si substrate and the breakdown voltage of the Si substrate. To increase the breakdown voltage of the nitride semiconductor layer, the thickness of the nitride semiconductor layer needs to be increased. However, Si and a nitride semiconductor considerably differ from each other in the lattice constant and the coefficient of thermal expansion, and thus the thickness of the nitride semiconductor disposed on the Si substrate is limited.
Thus, a Si substrate having a high breakdown voltage is under study to increase the vertical breakdown voltage of a semiconductor device (see, e.g., Japanese Patent Publication No. 2005-217049).